The objective of this proposal is to study the enzyme purine nucleoside phosphorylase from lymphoid cells and other human cells in an attempt to establish the relationship between enzymatic deficiency and immunodeficiency. The presence and the levels of the enzyme in separated T and B lymphocytes and monocytes from human peripheral blood will be determined as well as enzyme changes that may accompany stimulation of the above cells by mitogens and antigens. The enzyme will be purified from separated T and B lymphocytes, monocytes, blasts and macrophages and will be analyzed by two dimensional gel electrophoresis to determine the state of subunit modification. The composition of the subunits will be assessed by isoelectric and electrophoretic separation of each type of subunit of the erythrocytic enzyme in the presence of 8 M urea.